The use of ultrasonic transducers for determining flow rate is well known. For example, see U.S. Pat. Nos. 3,869,915, 3,987,674 and 4,232,548. Clamp-on ultrasonic transducer flowmeters are convenient to use because they do not require cutting a hole into a pipeline to determine the flow rate. They rely on the determination of sonic propagation velocity of ultrasonic energy emitted through the pipe wall into the fluid in the pipe to calculate the flow rate and can also be used to determine other characteristics or circumstances in a pipeline, for example, they can be used to determine if leaks have occurred.
In some ultrasonic flowmeter applications, however, it is desirable to use an insertion type transducer, i.e., one in which the mounting to the pipeline must be made through a hole in the pipeline. The situations in which an insertion type transducer are utilized are usually dominated by those in which very high temperatures are encountered, or wherein the pipeline contains very low sonic impedance media, such as gases.
The conventional state of the art transducer of the insert type is characterized as shown in FIG. 1, in which a coupling rod 10 is prepared with square cut end faces 20 and 30, on one end 30 of which a piezoelectric crystal 40 is mounted. The other end 20 is introduced into the pipe 50 through a cut to act as a radiative surface. Since it is necessary for the beam 60 to be emitted in an orthogonal relationship to the emitting surface 20, and to avoid reflection, the rod 10 is mounted at an angle to the pipe 50, generally around 45 degrees, as shown. In general, the rod 10 is not welded to the pipe 50 directly, but is normally mounted within an outer tubular mounting assembly 70, which is welded into place, as shown at 75. The weld typically acts as a sealing surface for the rod structure. Generally, an end cap 80 is bolted to the structure 70 to close off the structure 70 and also to allow the connection of wires 85 to the transducer. A seal 90 is typically provided between the rod 10 and the structure 70 at the cap 80. In some cases, arrangements are made for the rod 10 to be installed by means of a "hot tap" assembly, allowing for its installation to a pipe which is in operation and under pressure.
The difficulty with this type of transducer is that the crystal 40, which can operate either in the longitudinal or transverse mode, has a beam spread pattern 60 which hits the side wall of the rod as shown in FIG. 2A. The beam spread pattern 60 typically includes a primary longitudinal ray 63 whose axis is shown at 63A and several side lobes, e.g. as shown at 62. The energy distribution in the coupling rod 10 of the beam pattern 60 is shown in FIG. 2B. As shown, the side lobes 62 form a generally circular pattern concentric about the primary ray 63. In the case shown, the side lobes 62 of the beam pattern 60 usually include an angle, such as near 68 degrees for a longitudinal wave and 30 degrees for an injected shear wave, which generates conversion of a substantial percentage of the injected mode to the alternate mode, i.e., from longitudinal to shear, or vice versa. Thus, an injected beam generates a shear wave which contains much energy, since the side lobes 62 of the injected wave are substantial.
As shown in longitudinal cross section in FIG. 2A, a longitudinal ray 62A of side lobe 62 is first incident at the boundary of the rod 10 at an angle of approximately 68 degrees to the normal. The wave 62A is converted into a shear wave 62B traveling substantially transversely across the rod 10 to impact the boundary again further down the coupling rod 10, where it is remode converted to a longitudinal wave 62C. In the case of an injected longitudinal wave as shown in FIG. 2A, the twice mode converted wave 62C emerges from the rod late, since the converted shear wave 62B travels at a much lower propagation velocity than the longitudinal waves 62A and 62C. This generates a received signal at a receiving transducer assembly as shown in FIG. 3. The transmitted waveform is shown at 66 in FIG. 3 and the received waveform includes a primary longitudinal receive waveform from the primary longitudinal ray 63 which arrives early and has low amplitude, as shown at 68A and the mode converted wave form 68B which arrives late and has most of the energy. Such a multiplicity of receive signals, displaced in time, prevents generation of a fully coherent signal, and results in difficulty in identifying the exact time of arrival of the signal, resulting in flow computation errors, since the accuracy of ultrasonic flowmeters is highly dependent on determination of the exact transit time of the sonic beam through the fluid.
It has not been shown in FIG. 1, but a transducer assembly identical to that shown in FIG. 1 is typically mounted on the opposite side of the pipeline, in a line with the longitudinal axis of the emitted beam 60 as well known to those of skill in the art. The transit time for the some energy to go from one transducer to the other is normally determined and then used in the calculation of the sonic propagation velocity, which is then used to determine flow rate.